1. Field of the Invention
The present invention generally relates to image forming methods, image forming apparatuses and toner image patterns, and more particularly to an image forming method for calibrating a color registration error caused by a positional error of a plurality of color toner images that are formed on a transfer body, an image forming apparatus which employs such an image forming method, and a toner image pattern suited for use by such an image forming method.
2. Description of the Related Art
Image forming apparatuses typified by color copying machines and color laser printers include tandem type image forming apparatuses. In one example of the tandem type image forming apparatus, 4 color toner images of yellow, cyan, magenta and black are successively transferred from respective photoconductive bodies onto a transfer body, such as a transfer belt or a transfer sheet or medium (for example, paper). For this reason, a color registration error may occur if an error is generated in relative positions of the 4 color toner images. Because the color registration error greatly affects the quality of the color image that is formed by fixing the 4 color toner images on the transfer medium, it is important to minimize the color registration error in the tandem type image forming apparatus.
One example of a conventional method of calibrating the color registration error is proposed in a Japanese Laid-Open Patent Application No. 11-65208. According to this conventional method, toner images tmnY, tmnC, tmnK and tmnM (n=1, 2) for correction of the 4 colors yellow, cyan, black and magenta, are formed on a transport belt which transports a transfer medium in a transport direction A, as shown in FIG. 1. The toner images tmnY, tmnC, tmnK and tmnM are detected by an optical detection means, and positional errors among the toner images tmnY, tmnC, tmnK and tmnM are obtained from a detection result of the optical detection means. An exposure unit is controlled based on the obtained positional errors, by changing an exposure start time of the exposure unit, for example.
In the exposure unit which exposes a photoconductive body that is provided with respect to each of the 4 colors, laser beams from 4 laser light sources are reflected by reflection surfaces of a polygon mirror which rotates. An outer peripheral surface of each photoconductive body, which has a cylindrical shape, is exposed in an axial direction of the photoconductive body by a main scan of a corresponding laser beam. In addition, the photoconductive body rotates about its axis, which causes the outer peripheral surface of the photoconductive body to be exposed in a circumferential direction (that is, the transport direction A) by a sub scan of the corresponding laser beam. For example, in the exposure unit, the laser beams for exposing the photoconductive bodies that are provided with respect to the colors yellow and cyan are simultaneously reflected by one reflection surface of the polygon mirror, and at the same time, the laser beams for exposing the photoconductive bodies that are provided with respect to the colors black and magenta are simultaneously reflected by another reflection surface of the polygon mirror.
The toner images tmnY, tmnC, tmnK and tmnM for correction include first toner images tm1Y, tm1C, tm1K and tm1M made up of strips that have a linear portion forming an angle of 45 degrees with respect to both a main scan direction and a sub scan direction, and second toner images tm2Y, tm2C, tm2K and tm2M made up of strips that are arranged at predetermined intervals in the sub scan direction and have a linear portion parallel to the main scan direction, as shown in FIG. 1. However, because the toner images tmnY, tmnC, tmnK and tmnM for correction are arranged at both ends of the transfer belt along the main scan direction, the effects of errors, such as an error in an optical system of the exposure unit, appear conspicuously in terms of the positions where the toner images tmnY, tmnC, tmnK and tmnM are formed. Particularly, the first toner image tm1Y or tm1C that is formed by reflecting the corresponding laser beam by one reflection surface of the polygon mirror and the first toner image tm1K or tm1M that is formed by simultaneously reflecting the corresponding laser beam by another reflection surface of the polygon mirror shift in the main scan direction due to the effects of the errors. Consequently, depending on the error, the first toner image tm1C and the first toner image tm1K may be formed in an overlapping manner as shown in FIG. 2, for example, and in such a case, it becomes impossible to detect the first toner images tm1C and tm1K in a normal manner.